In a recent study, researchers had people track their workouts using "social gaming," where you earn prizes for working out. Not surprisingly, after 10 weeks, the people who played the games exercised more often, AND lost more weight.

If you're not sure what "social gaming" is, here are three examples. They're all apps, and you can download them for free on an iPhone or Android phone.

1. Nexercise. It rewards you for keeping track of your workouts. Every two minutes you spend exercising equals 10 points. Then you can redeem the points for things like Amazon gift cards. (Check it out on Nexercise.com.)

2. GymPact. This one actually charges you money if you don't work out. But if you DO, you WIN money. First you decide how much to penalize yourself for each workout you miss. Then you have to "check in" every time you exercise.If you DON'T, you lose your money for that day. But you get a few bucks when you DO meet your workout goals. And it all comes from users who DIDN'T work out that week. (Check it out at Gym-Pact.com.)

3. HealthRally. It lets your friends and family pledge money for rewards like iPads and running shoes. And you only get them if you reach your goal. If you DON'T, then none of their credit cards get charged. You can also pledge money to help friends with stuff like quitting smoking. (Check it out at HealthRally.com) (Men's Health)